1. Field of the Invention
The present invention relates to a system and method of measuring physiological signals of the human body, and more particularly, to a blood pressure measuring system and method.
2. Description of the Related Art
Physiological signals of a human body include information which can help in diagnosing the health of the human body. One such physiological signal is blood pressure, which, from a medical point of view, may be indicative of disorders of the circulatory system. When blood pressure is abnormal, an appropriate remedy may be taken after determining the cause of the abnormality.
Blood pressure can be classified into systolic pressure and diastolic pressure. The systolic pressure is measured when the heart ventricles contract, and the diastolic pressure is measured when the heart ventricles relax. When the heart ventricles relax, blood is not transported to arteries. Nevertheless, due to elasticity of the walls of blood vessels, blood inside the blood vessels is pressed somewhat, even though the degree of pressing is less than when the ventricle contracts. Accordingly, the diastolic pressure does not become zero.
Blood pressure may change depending on a variety of factors including, e.g., a patient's psychological state, measuring conditions, the environment, etc. For example, when a doctor or a nurse measures the patient's blood pressure, the patient's blood pressure may rise due to tension. Thus, it may be difficult to accurately measure the patient's blood pressure using only one measurement. Blood pressure measured when the patient has an empty stomach, just after getting up in the morning, may be referred to as the basal blood pressure, and may be very helpful for diagnosis. However, it may be troublesome to measure the basal blood pressure in real life. Medical centers, such as hospitals, may have difficulty assuring the conditions under which the basal blood pressure is to be measured, e.g., it may be difficult for the medical center to schedule patients early in the morning. The conditions under which the basal blood pressure is to be measured may be more easily met at home.
Various portable electronic blood pressure measuring devices, known as sphygmomanometers, have been developed and may be used in the home by the patient. Automated electronic sphygmomanometers capable of indirectly measuring the blood pressure have recently been developed. One type, hereinafter referred to as a conventional sphygmomanometer, employs a volume-oscillometric method to indirectly measure the blood pressure. Unlike auscultatory, or audible, sphygmomanometers, measurements made with a volume-oscillometric sphygmomanometer need not rely on a special converter or microphone.
FIG. 1 illustrates graphs of signals derived from a blood pressure measurement made using a conventional sphygmomanometer. A pressure signal G1 may be measured in a pressure cuff as a pressure applied to the cuff is reduced. Typically, the pressure cuff will be wrapped around an appendage, e.g., an arm, of a patient. An oscillating waveform G2, extracted from the pressure signal G1, represents pressure changes in the cuff related to pressure changes in an artery beneath the cuff, and is indicative of a pulse wave of the patient's blood pressure. The oscillating waveform G2 may be obtained from the pressure signal G1 by passing the pressure signal G1 through a 0.5 Hz high-pass filter. The volume-oscillometric method indirectly measures blood pressure by analyzing changes in the oscillating waveform G2, and uses that analysis to determine which points in the pressure signal G1 represent the patient's systolic and diastolic blood pressures.
Oscillations in the oscillating waveform G2 first appear when pressure in the cuff, represented by the declining pressure signal in graph G1, first drops below the patient's systolic blood pressure. At this point, the pressure in the cuff has decreased to the point where the patient's blood can begin to flow and pressure variations in the cuff caused by systolic pressure become evident.
It can be seen that the oscillating waveform G2 eventually reaches a maximum amplitude, which is indicative of the mean blood pressure MP. This value is generally easy to determine. In contrast to measurements made by the auscultatory method, however, it is more difficult to determine the systolic and diastolic blood pressures from the oscillating waveform G2. In particular, as the systolic pressure is indicated by the point at which oscillations first appear, the oscillating signal may be quite small at that point. Thus, automatically determining the point at which oscillations first appear may be difficult. In particular, noise in the signal may produce an error in determining where the oscillations first appear, which, in turn, may produce an error in determining what value in the pressure signal G1 represents the patient's systolic blood pressure.
Since the maximum amplitude in the oscillating waveform G2, the mean blood pressure MP, is generally easier to determine than the point at which oscillations first appear, the oscillometric method typically determines the systolic and diastolic pressures based on the mean blood pressure MP. Constant ratios are assumed between the value of the maximum oscillation in the oscillating waveform G2 and the values of the oscillations occurring in at the systolic pressure and the diastolic pressures, respectively. These ratios will be referred to as characteristic ratios. Thus, the volume-oscillometric method involves determining a maximum value of the oscillating waveform G2 and estimating the systolic and diastolic pressures based on the maximum value by applying the characteristic ratios to the maximum value. Tests have shown that the point in the oscillating waveform G2 that is representative of the systolic blood pressure corresponds to about 50% of the maximum oscillation, and the point in the oscillating waveform G2 that is representative of the diastolic blood pressure corresponds to about 50-80% of the maximum oscillation. Thus, a ratio of the systolic blood pressure SBP to the mean blood pressure MP and a ratio of the diastolic blood pressure to the mean blood pressure MP are the characteristic ratios.
In other words, the cuff pressure may be determined to be the systolic pressure where the amplitude SBP corresponds to 50% of the maximum amplitude MP. Similarly, the cuff pressure may be determined to be the diastolic pressure where the amplitude (not shown) corresponds to 75% of the maximum amplitude MP.
Unfortunately, the characteristic ratios may differ by 10-20% depending on differing body and biological characteristics of the patients and the cuffs used, e.g., differing external shapes of the cuffs, differing elasticity of the cuffs, differing pressure transfer characteristics in the patients' arteries or arms, differing viscosity and elasticity characteristics in the walls of arterial vessels, differing shapes and amplitudes of the blood pressure waveform of the heart artery, etc. Additionally, as the conventional sphygmomanometer measures the blood pressure at an upper arm, the patient may need to take off an upper garment and, since a large pressure may be applied to the arm of the patient, the patient may feel pain during repeated measurements.
A sphygmomanometer that can measure the blood pressure through a finger has been developed. Using the finger sphygmomanometer, the signals may be measured at an artery of the finger, which will typically be narrower than that of an upper arm. A signal-to-noise (S/N) ratio may be low and motion of the finger may have a great influence on the signals. Also, due to the difference in the transfer characteristic of the circulatory system, the blood pressure measured at the peripheral artery may be different from that measured at the upper arm. Accordingly, there is a need for an improved approach to measuring blood pressure.